1. Field of Invention
The present invention relates to a method and apparatus for making a filtering unit.
2. Related Prior Art
As disclosed in U.S. Pat. No. 7,575,678 issued to the present applicant, a portable water purification device includes a base, a fixed sleeve, a tube, a filtering unit, and a movable sleeve. The base includes a channel for providing filtered liquid. The fixed sleeve is fixed on the base and includes a first peripheral wall and a first axial hole. The tube, on which the filtering unit is mounted, is secured on the base and held in the first axial hole to define an aperture in communication with the channel. The movable sleeve is mounted on an external surface of the fixed sleeve, and slid between a first position away from the base and a second position adjacent to the base. The movable sleeve includes a second peripheral wall and a second axial hole in communication with the first axial hole to define a containing room when the movable sleeve is in the first position.
In the U.S. Patent, the filtering unit includes a screen sandwiched between two permeable membranes. The screen and the membranes are cut from substrates. Water-proof adhesive is provided along the edge of each of the screen and the membranes. Then, the screen and the membranes are located in a mold in which they are pressed. After the curing of the adhesive, the screen and the membranes become a laminate. Then, the laminate is punched with an aperture. Thus, the filtering unit is made. It however takes a lot of time for the adhesive to cure, and this is undesirable for massive production.
To expedite the production of the filtering unit, ultrasonic welding is used to bond the overlapped screen and membranes along the edges. Then, a knife is used to cut the bonded screen and membrane with an aperture. It is however difficult to execute the ultrasonic welding on the overlapped screen and membranes at one step where the edges are long. In general, each of the edges is divided into to three sections of marginally larger than 120 degrees, and the ultrasonic welding is executed at three steps. To make the edge well water-proof, ultrasonic welding is executed twice on a small portion of each of the edges between any two adjacent ones of the sections of each of the edges. Repeated ultrasonic welding would however cause excessive heating that would damage or deform the surface of the filtering unit. Such excessive heating would even make invisible but harmful slits in the filtering unit. Therefore, the yield is not satisfactory, and this is a heavy burden on quality control. Moreover, it requires a lot of laboring to execute the three-stepped ultrasonic welding, and this is hence inexpensive.
The present invention is therefore intended to obviate or at least alleviate the problems encountered in prior art.